Transcranial Magnetic Stimulation (TMS) is a method of causing current flow within the brain by electromagnetic induction, and stimulating neurons. According to this method, as shown in FIG. 1 to FIG. 3, by applying an alternating current or a given current waveform to a stimulation coil that has been placed above a person's scalp, a variable magnetic field is generated, and the effect of that variable magnetic field is to induce, within the brain, eddy current in a reverse direction to coil current, and nerve impulses is generated as a result of stimulation of neurons by this eddy current. This type of Transcranial Magnetic Stimulation is being used in clinical laboratory tests and cerebral function research, including measurement of nerve conduction velocity.
In recent years, magnetic stimulation has been gathering attention as a therapeutic application for neuropathic pain, Parkinson's disease, depression, etc. With these types of illness, there are cases where results are not witnessed with treatment using medicines. For example, for intractable neuropathic pain there is a method of treatment where electrical stimulation is given to the brain by implanting electrodes in the brain. However, this method of treatment requires a craniotomy, and so many patients are unwilling to have it performed.
Repetitive transcranial magnetic stimulation, where noninvasive magnetic stimulation, that does not require surgery, is repeatedly carried out, is therefore being researched as a method of treatment. With medical treatment for intractable neuropathic pain, it is being reported that pain relief effects have been attained at about one day after having carried out magnetic stimulation on the cerebral primary motor cortex.
However, a conventional magnetic stimulation device has a weight of about 70 kg, and at the time of installation electrical work is necessary in order to be able to supply electrical power from a 200 V power supply, which means that the device can only be used in well equipped medical facilities. Also, at the time of actual treatment, since it is necessary to determine stimulation position while referencing patient MRI data in accordance with the disorder to be treated, medical treatment by a medical worker who is experienced with that situation is necessary. With the treatment of intractable neuropathic pain, it is necessary to carry out positioning of a coil on the primary motor cortex, which constitutes the target, in units of 1 mm.
With transcranial magnetic stimulation therapy, as a stimulation coil for magnetic stimulation, currently various forms have been proposed, including a circular coil and a FIG. 8 coil (a coil that is wound more or less in the shape of the number “8”), and further a quatrefoil coil, a Hesed coil, and a coil having multiple small circular coils arranged on the surface of a head section, and currently the circular coil and figure 8 coil are mainly being utilized.
A figure 8 coil (refer to patent publication 1 and patent publication 2 below) has two circular coils, formed in series using a single conductor, arranged partially overlapping, and by having electrical current flow in opposite directions in these circular coils it is possible to cause eddy currents to converge directly beneath a section where the coils cross, and stimulate a local region.
On the other hand, depending on the object of treatment or on the personal symptoms of the patient, there may be cases where instead of localized stimulation, stimulation over a wider range is effective.
Also, with a coil that focuses stimulation locally there is a need to accurately determine position on the target region, and in this case it is necessary to implement accurate positioning using a navigation system or the like.
As well as carrying out development of magnetic stimulation used in home treatment, there has also been advancement in development of navigation systems for determining stimulation position by a non-medical worker. According to the system, first a patient is fitted at the hospital with glasses having a magnetic sensor, and calibration is carried out using a permanent magnet in order to attach the glasses at the same position every time. Next, a doctor specifies optimum stimulation position using a procedure that combines a patient MRI image and an optical tracking coordinate system, and the optimum stimulation position, and data for random positions in a range of 5 cm around the optimum stimulation position, are stored. By storing surrounding position data, it is possible to for the patient to visually know where a coil currently is when determining coil position.
At the time of home treatment, first calibration of the glasses is carried out. After that, three-dimensional position is measured by comparing position of permanent magnets that are fitted to the stimulation coil with data. By visually confirming current position of the coil and optimum stimulation position, it is possible to instinctively carry out positioning of the coil.
By experimentation it is found that navigation error of this navigation system is a maximum of, for example, 5 mm from the optimum stimulation position, while on the other hand if the figure 8 coil that was described previously has an irradiation position (optimum stimulation position) within this 5 mm, it is possible to provide therapeutically effective stimulation of the target region. This means that at a stimulation position that has been guided by using a navigation system, if a treatment device that carries out magnetic stimulation with a figure-8 coil is used, there is a possibility that a region that is to be radiated (optimum stimulation position) will not be within the effective stimulation range of the treatment coil, and so it will be difficult to accurately carry out stimulation to the treatment region. Accordingly, it is necessary to develop a coil that is capable of generating eddy current uniformly over a wider range, such that in a case where there is a region to be radiated within, for example, 10 mm, a target region can be stimulated in a therapeutically effective manner.
Therefore, in order to implement a stimulation coil having high robustness (specifically, being capable of generating uniform eddy current over a wider range), a dome type coil device (in the specification below, referred to as “dome type coil”) has been proposed by the present inventors (refer to patent publication 3 below). This dome type coil can cause eddy current to be generated over a wide range compared to the figure 8 coil, and there is also the desirable property of being able to reduce inductance while maintaining inducement of eddy current over a wide range.
However, while the dome type coil shown in patent publication 3 below can generate an induced electrical field over a wide range compared to the figure-8 coil, as already stated, there is a problem in that electrical field intensity is low in a case where the same electrical current as with the figure-8 coil has been applied (approximately ¼ under the same current application conditions).
In a case where induced electrical field is small, more electrical current must be applied in order to compensate for this, which means that not only is there a possibility of device cost and installation cost being increased due to boost circuits and capacitors being increased in size, there was also a problem in that that coil itself heats up rapidly and it is necessary to take measures to deal with this.
Accordingly, the present inventors have carried out various experiments regarding coil shape and design parameters, and as a result have acquired knowledge regarding shapes that have the advantage of being able to provide the same wide induced electrical field as a dome type coil while being able to generate a stronger induced electrical field with approximately the same applied current, and that can comprises a coil that does not obtain a value of inductance that has deviated.